Following the 2012 London Olympics it was decided to commemorate all the British Gold Medal winners by repainting a pillar box in each of their home towns gold. This box isn't dedicated to one particular winner, but to the City of London itself, as the host city.
The cache itself is not at this pillar box. To find it read the name of the MP who unveiled the plaque, which can be found on the box. The number of letters in their first name = A, and their second name = B.
The cache can be found at N51 29.(A+B)(A+1)(B-B) W000 07.B(A*4)(B+1)
If you are interested in unusual pillar boxes, then check out my other caches in this series:
Special Delivery #1 - GCACP55
Special Delivery #2 - GCAR9WC
Special Delivery #3 - GCARPBA
Special Delivery #4 - GCAVWE1
Special Delivery #5 - GCAXDX6
Special Delivery #6 - GCB4GFY
Special Delivery #7 - GCB118Y
Special Delivery #8 - GCAXE0N
Special Delivery #9 - GCB2ZV0
Special Delivery #10 - GCB359X